The UK will more than double its fleet of remotely piloted drones in a bid to step up the fight against Islamic militants and bolster security in Britain, David Cameron has announced. The prime minister, speaking at the start of the Conservative's annual conference in Manchester on 4 October, also revealed that his government would upgrade equipment provided to the UK's Special Forces, including the SAS and the SBS.

The move means that the number of RAF drones – otherwise known as Remotely Piloted Air Systems (RPAS) – will surpass 20 and will be upgraded with the latest technology. The government also claimed the increased fleet will substantially enhance the UK's global intelligence, surveillance and reconnaissance capabilities.

"National security is the first priority of this government. We are taking action both at home and abroad so we address terrorism from every angle," Cameron said. "As part of the defence review, we will be more than doubling our drone fleet. This will be combined with increasing the capacity of our special forces, so that the country remains ready to address any threats to our nation's security."

Legal action

The upgrade announcement comes after Cameron revealed in the House of Commons that an RAF drone strike had killed two Islamic militants from Britain – Reyaad Khan (21) and Ruhul Amin (26) – in Syria. The Raqqah attack, which was launched on 21 August, was controversial because Cameron had previously promised to put a vote before parliament over military intervention in the Middle Eastern country.

But Cameron told MPs on 7 September that the move was "an act of self-defence" and said that he had consulted the Attorney General, Jeremy Wright QC, over the legality of the strike.

"We should be under no illusion. Their intention was the murder of British citizens. So on this occasion we ourselves took action," he said. "I can inform the House that in an act of self-defence and after meticulous planning Reyaad Khan was killed in a precision air strike carried out on 21 August by an RAF remotely piloted aircraft while he was travelling in a vehicle in the area of Raqqah in Syria."

But the British government now faces legal action from law firm Leigh Day, human rights charity Reprieve, and Green Party leader Caroline Lucas and grandee Baroness Jenny Jones.

"The government appears to have adopted a 'Kill Policy' in secret – without parliamentary debate or the prospect of proper independent scrutiny. Sanctioning lethal drone attacks on British citizens is a significant departure from previous policy, as well as potentially unlawful, and it's deeply concerning that it has occurred without appropriate oversight," Lucas said.

"By refusing to publish the legal basis for these attacks, the government has created a legal and accountability vacuum," she stated. "We need to be able to determine whether the attacks – and what they signify in terms of government policy – meet the robust conditions set out in international and domestic law."